HDR footage from iPhone combined with non HDR jpg

Ann, as always, is right!

Three things must match to get an expected "proper" export:

• Clip color management color space setting;
• Sequence color management color space setting; and
• Export color management color space settings ... many of them!

On the timeline, Premiere takes the original clip data, any changes made, applies essentially an EOTF (electro-optical transfer function) to convert to chosen display space, you see the image.

For exports, Premiere takes the original clip data, any changes made, doesn't add the EOTF ...  and makes the export. Hence suddenly a blown out image.

Auto-log detect and auto-tonemapp do affect all this somewhat ... but ... if you don't have the clip, sequence, and export CM matching, you may well get surprises. Like having some clips come through as expected, others not.

I do not recommend modifying presets to make a different color space/dynamic range than the preset was built for. As there are a ton of picky, often nearly hidden, choices ... and all must be correctly set or it goes bonkers.

For SDR/Rec709 exports, always use the 'standard' presets that don't list either HLG or PQ in the preset name.

For HDR exports, always start with a preset with the appropriate HDR space in the preset name.

Period.

Everything needs to be the same: footage, sequence setting and export settings.